1. Field of the Invention
The present invention relates to a system and method for printing an image onto a medium using a coating solid to interact with the printed image. More particularly, the present invention relates to an inkjet printer system including inkjet printing and coating a solid onto a medium to improve print quality and image release when the coating solid interacts with the jetted ink.
2. Description of the Related Art
FIG. 1 illustrates an example of a conventional inkjet printer, including inkjet printer 10 having a platen or tray for receiving media, such as paper, a carriage 15 to carry ink cartridge(s) 17, and a processor 20 to control the operation of inkjet printer 10. As illustrated in FIG. 1, carriage 15 carries ink cartridge(s) 17 across media or potentially axially across an intermediate transfer medium (ITM) 40, as further illustrated in FIG. 2. In FIG. 2, an ink cartridge travels across ITM 40 while printing onto ITM 40 in spiraled swaths, i.e., helical printing. Once the printing to ITM 40 has completed, media 30 is compressed between a nip generated at the intersection of ITM 40 and roller 45, thereby transferring the image printed on ITM 40 to media 30.
Although the example illustrated in FIG. 2 shows helical inkjet printing onto ITM 40, many alternative techniques are also available, such as printing directly to media 30, without an ITM, or printing onto media 30 while mounted on a media support medium.
FIG. 2 also illustrates that a liquid coating may be applied to the surface of ITM 40. The liquid coating is used to improve print quality, as well as release and transfer of an image from an ITM to media. To provide the improved image quality, liquid coating compositions may destabilize a colorant in ink prior to penetration into the media. The colorant in ink might be dyes, pigments, or other materials, depending on the chemical structure of the ink. Similarly, the liquid coating composition is designed to interact with a corresponding ink. For example, the liquid coating may be a flocculent, such as a liquid that contains a multivalent salt or is low pH, which may be applied to the media or ITM before, during, or after the jetting of ink. When the ink impacts the flocculent, the colorant in the ink destabilizes, thereby preventing penetration of the colorant into the media while allowing penetration of the remaining ink constituents. In another example, media can be pre-coated with a liquid composition that contains a flocculent. Further, in this example, a mordant may also be added to the liquid composition to reduce spreading and color-to-color bleeding of the ink.
The coating material is only applied and resident on the media or ITM in a liquid form, which thereby introduces extra water into the media. In addition, a liquid coating composition may increase paper cockle, and a coat thickness can easily vary by more than 100%.
Depending on the viscosity of the fluid coating material, the application of a uniform layer of a particular thickness of fluid can be very difficult to accomplish. If the coating fluids are very thin, then foam rolls or felt wicks can be used to apply coatings. Very thin fluids can also be jetted via inkjet-like print heads. If fluids are thicker or of a higher viscosity (to allow the use of chemicals which provide bigger print quality improvements or more rapid ink absorption effects), then more complicated application methods such as blade coating or roll coating become necessary. These methods are challenged to obtain uniform coatings at reasonable power consumptions, especially at process speeds desirable for ITM printing applications. It is especially difficult to distribute fluid uniformly across the width of a page-wide blade coater in order to produce a uniform coating. On the other hand, traveling blade coaters (those that are not page-wide) reduce throughput and increase machine width for drum printers, and are quite complicated to make operate bidirectionally. Blade coating methods also produce coating thicknesses which are highly speed dependent, which is a major limitation for printers which operate at more than one process speed (i.e. to produce outputs at different print resolutions).
In addition, the liquid coating material for ITM printers can have undesirable interactions with jetted liquid ink droplets, allowing the droplets to move or grow in size on the ITM rather than remaining fixed in place. However, the requirement to maintain flowability of the liquid coating material can limit the availability of the active chemical components, or concentration of the same, that can be included in the liquid coating material.
Examples of ITM printing systems using liquid coatings are described in U.S. Pat. Nos. 5,389,958, 5,805,191, and 5,677,719, all of which describe liquid coating material on an ITM, jetting ink onto the liquid coated surface of the ITM, and thereafter transferring the ink image to media through a nip generated by the ITM and a roller. Liquid coating systems require fluid handling hardware, including subsystems to store fluids, to move them from the storage vessel to the coating system, to apply them to an ITM, and to clean off residue after image transfer. These subsystems also have issues with fluid containment, which may restrict the orientation of printers during use or shipping. Liquid coating systems have been used to improve print quality for inkjet printers. As noted above, the liquid coatings usually interact with components of the ink, flocculating pigment particles, fixing dyes, or affecting absorption of ink components into the media, for example. Examples of such liquid coating techniques have also been illustrated in U.S. Pat. Nos. 6,183,079 and 6,196,674.
The present inventors have concluded that rather than applying liquid coatings, it would be more advantageous to apply coatings in a solid form. In addition, it is difficult to control the application of the liquid coating layers for thin even coating layers, while the application of a coating solid layer does not suffer from this limitation. Thus, a previously unknown method and apparatus for application of a coating solid layer, performing destabilization of a colorant in an ink, would appear to be necessary.
U.S. Pat. No. 6,059,407 describes a process where efficient transfer of an ink image from an ITM is accomplished with a particular applied transfer drum material and a solid surfactant. In U.S. Pat. No. 6,059,407, several different low surface energy rubber materials were used, each providing for highly efficient release of the ink image from the ITM. However, print quality defects resulted from the low surface energy of these particular rubber materials, with the ink image moving and flowing significantly on the surface of the low surface energy rubber materials. To counter this effect, this U.S. Pat. No. 6,059,407 describes applying a surfactant, in a solid form, to the surface of the drum, with the surfactant having an HLB (hydrophilic-lipophilic balance) value between 2 and 16. HLB is a reference value to compare different surfactants in a relative sense. The actual value needed would be dependent on drum surface and ink formulation. U.S. Pat. No. 6,059,407 also describes most of the classes of surfactants available.
However, the sole purpose of applying the solid surfactant in this U.S. Pat. No. 6,059,407 is to control the spread of the ink image on the surface of the ITM, caused by the unique low surface energy rubber materials. Thus, the surfactant does not aid in the transfer efficiency of the ink image to the media, e.g., performing destabilization of a colorant in an ink, but rather, merely compensates for a low surface energy aspect of the unique transfer drum materials. In addition, after application of the solid surfactant material, it would appear that the surfactant material is liquefied into a liquid layer while on the surface of the transfer drum.
Conversely, the purpose of the liquid coatings, and the inventors' coating solids, is to effect efficient transfer of ink colorant to the media. Typically, surface energy modifications are not necessary to maintain print quality. Rather, if image spread is observed, it can be modified within the ink formulation. Embodiments of the present invention may not even include any surfactant in their solid material formulations, while also noting that surfactants may not diffuse with an ink on a time scale required in printing systems to perform this destabilizing operation.